Ok, me and a friend of mine are 11/11 with Colo and we've been doing it with only 2. I use a Bst/War, and she uses a Rdm/Whm, both Elvaan. The first time we did it we set the record at 7 mins 58 secs. We've broken the record we set a few more times since then and it is now at 5 mins 30 secs.
Items used:
Bst: 3 Hi-Potions, 1 Fish Mithkabob (or Boiled Crab), 1 Pamama Au Lait or Persikos Au Lait.
Rdm: 3 Hi-Potions, 2 Yagudo drink, 1 Pumpkin or Rolonberry Pie.
Most of the time we don't use the Hi-Potions, but the difficulty of the battles varies, and there were a couple times I used all 3. Sometimes it takes a few tries to charm one of the Furies, and that can wear you down fast as well.
Strategy:
Make sure the Bst has at least +8 Chr and around 100 Def with no buff. Enter the battlefield and use the Fish Mithkabob/Boiled Crab. Wait for Rdm to buff and use Pie and Yagudo Drink. Wait for Rdm to heal to full MP then as soon as Rdm full, run and try to charm one of the Furies. If you fail, then try again. If you fail again the try the other Furie (It seems that when I fail twice on one Furie, the other Furie charms the first try). After you sucessfully charm one of the Furies, have it attack the other Furie. Now attack Colo and use Provoke on him. After you get Colo on you and are attacking, use 2-hr "Familiar". Rdm should cast Dia on the un-charmed Furie and Colo, then proceed helping you attack Colo while curing when needed. Colo will usually be at around 1/4 health when one of the Furies finally dies. If it was your familiar that died, try charming the other Furie while still attacking Colo. When you get it charmed have it help you attack. Colo will probably die before the charm wears off, then finish off the Furie. If your familiar is the Furie that survived, have him attack Colo and you and rdm need to stop everything. Your familiar needs to get hate from Colo so it can die. If your familiar is the last mob standing, you will lose because the charm will not wear off until way past the 15 minute time limit;).
This strategy has worked for me 11 out of 11 times. I only tried it becuase we got tired of waiting for a LS member who was gonna be a 3rd member. Now when he does show up again he's SOL =P.
Drops to date: 4 Thunder Spirit Pact, 1 Platoon Lance, 3 Gunromaru, 3 Ganko, 2 Platoon Sword, 1 Platoon Dagger, and quite a few lesser value items...misc rocks, scrolls, and such.
I'm glad to say that we are the current record holders on Diabolos with 5 mins 30 secs, and we did it with 2 instead of 3 ppl:) I've farmed this BCNM enough so now i'll share my tactics, good luck to you. If anything seems unclear post a reply and I'll try to answer any question.
Update:
In my 10th battle, I accidently let Colo die and I still had my familiar, the one I used 2-hr on, so I was basically screwed since the charm wouldn't wear off until the time limit was up...but I had a thought. I thought, "What if I log out and back in?"...It worked. When I logged out the bat was no longer charmed and proceeded to slap my Rdm around while I was gone, when I got back I simply joined back up with her and killed the Furie...got the Armor Box again:)